1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device produced by depositing a non-monocrystal silicon (called Si hereinafter) film on an insulating film formed on an insulating substrate made of glass or the like or a variety of substrates. For example, the method is suitable for making a thin film transistor (TFT), a thin film diode (TFD) and a thin film integrated circuit containing transistors and diodes, especially a thin film integrated circuit for an active liquid crystal display (LCD).
2. Description of the Related Art
Recently, a semiconductor device in which TFTs are mounted on an insulating substrate made of glass or the like, for example an active liquid crystal display and an image sensor in which TFTs are used for activating pixels. has been developed.
Generally, thin film Si semiconductors are used for the TFTs used in the above-mentioned devices. The above-mentioned thin film Si semiconductor comprises two types of semiconductors, these being an amorphous Si semiconductor (a-Si) and a crystalline Si semiconductor. The amorphous Si semiconductor is used most generally because it can be readily produced at low temperatures by a vapor phase process, and is suitable for mass production; however, it is less conductive than a crystalline Si semiconductor.
Therefore, it is strongly desired that a fabricating method of TFTs made of a crystalline Si semiconductor should be established which can hereinafter obtain a higher speed characteristics. As a crystalline Si semiconductor, polycrystalline silicon, microcrystalline silicon, amorphous silicon containing a crystalline component and a semi-amorphous silicon which is in an intermediate condition of a crystal and an amorphous are well known.
As the production method of a thin film crystalline Si semiconductor. the following methods are known:
(1) Crystalline film is deposited directly.
(2) First, amorphous semiconductor film is deposited and next, it is crystallized by the energy of a laser beam.
(3) First, amorphous semiconductor film is deposited and next, it is crystallized by applying thermal energy for a long time (annealing).
However, even formation of a film which has a good semiconductive characteristic on the overall surface of a substrate by the method described in (1) is technically difficult. This method also has a cost problem in that a low-priced glass substrate cannot be used because the film is formed at a temperature of 600xc2x0 C. or more. Deposition of a film which has a good characteristic at low temperatures by this method is difficult. As crystals grow perpendicularly to the substrate, film formed by this method is not suitable for TFTs which have flat conductivity.
For example, if an excimer laser used most generally at present is used in the method described in (2), this method has the problem that throughput is low because the area on which the laser beam is radiated is small. This method also has another problem in that the stability of the laser is not sufficient to form film evenly on the overall surface of a substrate having a large area. Further, this method requires a substrate to be heated and irradiated by a laser in a vacuum in order to crystallize well. Therefore, this method has the problem that throughput is limited.
The method described in (3) has the advantage that a substrate having a large area can be processed by this method compared with the methods described in (1) and (2). However, this method also requires high temperatures of 600xc2x0 C. or more to heat a substrate on which amorphous film is formed. If a low-priced glass substrate is used, the heating temperature must be lowered. Especially at present, LCD screens are becoming larger and larger and therefore, a large-sized glass substrate is required to be used for such large screens. If a large-sized glass substrate is used, a significant problem of shrinkage or distortion caused in the heating process essential for producing semiconductor devices and which deteriorates the precision of mask alignment occurs. Especially if a substrate made of No. 7059 glass manufactured by Corning Inc. used most generally at present is used, distortion occurs at a temperature of 593xc2x0 C. and significant deformation occurs in the prior crystallizing process due to heating. The heating time required for crystallization in the conventional process exceeds 20 to 30 hours and therefore, reduction of the time is also required together with reduction of the heating temperature.
The object of the present invention is to provide a means of solving the above-mentioned problems. In detail, the object is to provide a process in which good crystallization is obtained at lower heating temperatures, in other words, the effect on the glass substrate is reduced where a method by which film made of amorphous silicon is crystallized by heating is used.
Another object of the present invention is to provide a means of reducing or removing a metal element (a catalytic metal element) added to silicon film to promote crystallization.
In the first process according to the present invention, a crystallized silicon film is heated selectively by irradiating strong light on the surface of a Si film crystallized by a metal element which promotes crystallization in an ambient atmosphere containing chloride gas such as hydrogen chloride (HCl), carbon tetrachloride (CCl4) and silicon tetrachloride (SiCl4), or fluoride gas such as nitrogen trifluoride (NF3) and dicarbon hexafluoride (C2F6) to 10 to 90%. Producing plasma by excitation of a microwave or a high frequency has an effect of promoting the reaction during irradiation with strong-light.
If strong beams, for example light between near infrared rays and visible light, preferably light from 0.5 to 4 xcexcm in wavelength (for example infrared rays with peaks at 1.3 xcexcm in wavelength) are irradiated according to the present invention, it is desirable that beams are irradiated only for a relatively short time of approx. from 10 to 1000 seconds and that the surface of silicon film is heated until it is at from 900 to 1200xc2x0 C. As the light in the above-mentioned wavelength is absorbed by a silicon film and is not absorbed by a substrate substantially, selective heating of the Si film is enabled without having an effect on a substrate if beams are irradiated only for the above-mentioned time.
Visible light, especially light in wavelength of 0.5 xcexcm or less is absorbed well by intrinsic or substantially intrinsic amorphous silicon and can be converted to heat. Near infrared rays or visible radiations from 0.5 to 4 xcexcm in wavelength are absorbed effectively by intrinsic or substantially intrinsic crystallized silicon film in which phosphorus or boron is contained only to 1017 cmxe2x88x923 or less and can be converted to heat. On the other hand, far infrared radiation 10 xcexcm or more in wavelength is absorbed by a glass substrate and can be converted to heat. However, if most light are light 4 xcexcm or less in wavelength, little light is absorbed by glass. That is, near infrared rays or visible radiations from 0.5 to 4 xcexcm in wavelength are suitable for heating crystallized Si film formed on a glass substrate selectively.
If ultraviolet rays of which wavelength is shorter than the above-mentioned light are used, they are absorbed not only by Si film but by most substrate materials and therefore, the most suitable time for irradiating light should be shorter. For example, in case of light 248 nm in wavelength, it is desirable that the above-mentioned time is 1 xcexcsec. or less. If the above-mentioned light is irradiated for a longer time than the above-mentioned time, much light is absorbed by the substrate, which causes deformation of the substrate. As described above, the amount of light must be selected so that the temperature on the surface of the Si film exceeds 1000xc2x0 C. temporarily by irradiation of light for an extremely short time. The first irradiation cannot oxidize Si film fully because rise or fall of temperature on the surface of Si film is momentary. Therefore, multiple irradiations are required. In this case. the thickness of the formed oxide film depends upon the number of irradiations.
It is ideal that a pulse oscillating laser such as excimer laser is used as a light source to irradiate with ultraviolet rays for the above-mentioned extremely short time. A variety of excimer lasers emit laser light 100 nsec. or less in pulsewidth. A light equivalent to a laser light may be used.
When the temperature of a substrate was at 600xc2x0 C. or less, preferably 400xc2x0 C. or less while film formed on the substrate was irradiated by strong beams according to the present invention, the effect of oxidation was enhanced.
By the high temperature described above, a metal element in the Si film reacts with chloride or fluoride gas in an ambient atmosphere though the reaction is performed only for a short time and chloride or fluoride metal is formed on the surface of the Si film. As its boiling point is low, it is vaporized in the ambient atmosphere to reduce a density of the metal element in the Si film. Further, after the strong beam irradiating process is finished, the formed chloride or fluoride metal can also be removed by cleaning the Si film with pure water fully. However, the other elements contained in the Si film, such as the alkaline elements sodium and potassium are also removed at the same time by the above-mentioned process.
In such annealing, the Si film is often peeled because of the difference in coefficient of thermal expansion between the Si film and the substrate and in the temperature gradient between the surface of said Si film and the interface between said substrate and said Si film. If a film covers the whole or a large area of the substrate, peeling of the film is especially remarkable. Therefore, it is desirable that peeling of the film is prevented by dividing the film into satisfactory small areas or leaving the space between films sufficiently not to absorb extra heat. As the whole of a substrate is prevented from being heated through the Si film according to the above-mentioned way, shrinkage of the substrate by heat can be restrained to the minimum.
As the temperature of the film is raised by irradiating strong light, crystallization of the Si film is also promoted more as the secondary effect of the present invention. It has been observed that needle-shaped crystals grow not in the direction of the film thickness but in the direction along the surface of a substrate in Si film crystallized by addition of nickel. The width of the above-mentioned needle-shaped crystals is approx. 0.5 to 3 times of the thickness of the Si film and they grow little laterally, that is, in the direction of the side of them. Therefore, amorphous areas or areas which are crystallized only at the same extent as them are left between crystals. In the above-mentioned amorphous areas, crystallization is not completed even by annealing for a long time and if such a semiconductor is used for TFT, a problem that the characteristic of TFT cannot be enhanced fully occurs.
As high temperature of 600xc2x0 C. or more can be obtained by the process of irradiating strong beams according to the present invention, the present invention contributes to promotion of further crystallization of the above-mentioned areas which are crystallized only to the low extent. That is, it is because according to the present invention crystals grow epitaxially in the sides of needle-shaped crystals to crystallize amorphous areas.
When strong light is irradiated according to the present invention, many dangling bonds of silicon may be formed by thermal energy. These dangling bonds can be reduced (i.e. neutralized) by thermally annealing in an atmosphere containing hydrogen at a temperature of from 200 to 450xc2x0 C. and therefore, the characteristics and reliability of various semiconductor devices, for example, a thin film transistor (TFT) and a thin film diode (TFD) can be enhanced.
In the second process according to the method of the present invention, oxide is formed on the surface of heated Si film crystallized by a catalytic metal element in ambient oxidizing atmosphere containing chloride or fluoride gas to 10 to 90%. For the temperature of oxidization, the temperature at which a substrate does not warp or shrink is desirable. For example, an oxide film a thickness of from 40 to 100 xc3x85 is formed on the surface of the film at a temperature of 550xc2x0 C.
At this time, producing plasma by excitation of a microwave or a high frequency has an effect of promoting the reaction. Oxide can be formed not only by heating but by irradiating strong light in the above-mentioned atmosphere.
Metal elements existing in Si film, especially near the surface in large quantity are absorbed into the oxide selectively by formation of the oxide film. described above. Then, such metal elements can be removed or reduced by removing said oxide film in an etching process. In this process, thermal or light energy can be given to the Si film both by thermal oxidation and by oxidation by irradiating strong light and at the same time the crystallinity of the Si film can be improved.
When after such a strong beam irradiating process annealing is performed again, the effect of the present invention can be further enhanced.
According to the present invention, as described above, a catalytic metal element in silicon film is deposited on the surface of an Si film as chloride or fluoride, vaporized or trapped in oxide in large quantity and etched. As a result, the density of metal elements in the Si film can be reduced to up to one fifth or less of that before processing. At the same time, a Si film can be heated by irradiating strong light or by heating during oxidation and crystallization can be enhanced. A glass substrate absorbs few infrared rays even if strong light is irradiated and therefore, annealing by light can be performed without heating so much that the glass substrate cannot be used industrially because of softening or shrinkage.
In the third process according to the method of the present invention, strong light or laser light is irradiated on non-monocrystal semiconductor film crystallized by heating at a temperature of 600xc2x0 C. or less in order to promote crystallization further. At the same time by irradiating strong light or laser light, the quality of said film is also condensed. The third process is characterized by the above-mentioned condensation of quality of the film. In detail, in said third process, an Si film is heated by irradiating a laser light on said silicon film or by irradiating light between near infrared rays and visible light, preferably light 0.5 to 4 xcexcm in wavelength, for example, infrared rays with peaks at 1.3 xcexcm in wavelength for a relatively short time of approx. from 10 to 1000 seconds on said silicon film. The heating promotes crystallization. The third process is characterized by the promotion of crystallization. It is desirable that the wavelength of the light used is absorbed into an Si film but not absorbed into a glass substrate substantially. Further, in such annealing, the Si film is often peeled because of the difference in the coefficient of thermal expansion between an Si film and a substrate and from the temperature gradient between the surface of the Si film and the interface between the substrate and the Si film. If the film covers the whole of a substrate, peeling is especially remarkable. Therefore, peeling of the film can be prevented by dividing the film into fully small areas and leaving enough space between the films not to absorb extra heat. As only a part of the surface of a substrate is heated through the Si film, shrinkage of said substrate caused by heat is restrained to the minimum.
The present invention comprises the first step in which a crystalline Si film is produced by annealing, the second step in which the processing related to patterns for the Si film is performed and the third step in which the Si film is heated by strong light. Between the second and the third steps, insulating film used in the third process which absorbs no light (that is, transmits the strong light) may be formed on the Si film. The insulating film may be made of silicon nitride or silicon oxide. For irradiation performed in the third process, laser light may be used.
Visible light, especially light shorter than 0.5 xcexcm in wavelength is absorbed well by intrinsic or substantially intrinsic amorphous silicon and can be converted to heat. In a process according to the present invention, light 0.5 to 4 xcexcm in wavelength is used for irradiation. The light in the above-mentioned wavelength is absorbed effectively by intrinsic or substantially intrinsic crystallized silicon film (containing phosphorus or boron 1017 cmxe2x88x923 or less) and can be converted to heat. Extreme infrared rays 10 xcexcm or more in wavelength are absorbed by a glass substrate and said substrate is heated, however, if most light are 4 xcexcm or less in wavelength, glass is heated little. That is, light 0.5 to 4 xcexcm in wavelength is required to crystallize the crystallized Si film further.
In the case where crystallization utilizing a metal element such as nickel which promotes the crystallization of the crystalline silicon is employed as the above-mentioned first step of the present invention to realize the crystallization at a temperature lower than the temperature of the normal solid phase crystal growth, the effect of the present invention is remarkable. For elements usable for the present invention with which crystallization is promoted, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt belonging to the eighth family of elements may be used. Sc, Ti, V, Cr, Mn, Cu and Zn belonging to 3d elements may also be used. Further, it has been verified by experiments that Au and Ag also promote crystallization. Among the above-mentioned elements, it is Ni that has a remarkable effect and the operations of a TFT have been verified using a crystalline silicon film crystallized with Ni.
It is observed that in Si film to which any of the above mentioned metals is added, needle-shaped crystals grow not in the direction of film thickness, but in the direction along the surface of a substrate. However, the overall surface is not crystallized evenly and amorphous areas or areas in which crystallinity is low to the same extent as amorphous areas are left between crystals.
As described above, in Si film to which any of these metal elements is added, crystals grow in the shape of a needle, however, they grow little laterally, that is, in the direction of their sides. The width of the above-mentioned needle-shaped crystals is approx. 0.5 to 3 times of the thickness of Si film. Therefore, in said amorphous areas, crystallization is not completed even by annealing for a long time. The characteristics of a TFT are not enhanced fully if a semiconductor film with amorphous areas described above is used for the TFT.
The third step according to the present invention especially contributes to promoting further crystallization of areas in which crystallinity is low between needle-shaped crystals like teeth of a comb by heating at a temperature of from 800 to 1300xc2x0 C. (measured by bringing a thermocouple into contact with silicon). It is because crystals grow epitaxially in the direction of the sides of needle-shaped crystals and amorphous areas are crystallized.
The part except active elements, for example, the areas on which a thin film transistor (TFT) is formed, of a thin film Si semiconductor crystallized by annealing at a temperature of from 400 to 650xc2x0 C., typically at from 500 to 600xc2x0 C. is removed in the pattern transfer and etching process. Silicon film can be heated selectively to promote further crystallization by irradiating visible radiation or near infrared rays to crystalline areas scattered like islands in which needle-shaped crystals are grown laterally. At this time, as a glass substrate or the like absorb little infrared rays, annealing by light can be performed without heating to the extent that said glass substrate cannot be used industrially because of softening or shrinkage.
Especially, if a metal element which promotes crystallization is used in annealing, crystallization in the direction of the sides of needle-shaped crystals which is not enough so far is promoted and very dense crystalline semiconductor thin film can be produced. Embodiment 1 described below shows crystalline areas in Si film are increased according to the present invention, for example, using Raman spectroscopy.
Embodiments 2 to 4 show examples of processes in which TFT is manufactured according to the present invention.
A method of fabricating a semiconductor device in accordance with another aspect of the present invention comprises:
forming a non-monocrystal silicon film containing a catalytic element therein on a substrate;
crystallizing the non-monocrystal silicon film by thermal annealing;
forming a pattern of the non-monocrystal silicon film in the shape of an island;
promoting crystallization of the non-monocrystal silicon film by radiating a light thereto;
forming a gate electrode on the pattern;
introducing an impurity into the pattern with the gate electrode as a mask; and
activating the impurity by heating.